The present invention relates to rolls which can be used to transport and/or to otherwise treat sheet-like materials. Typical examples of rolls to which the present invention pertains are the rolls (or at least certain rolls) of calenders, embossing devices and similar machines wherein two or more rolls cooperate to subject a web of paper textile, synthetic plastic or other material to compressive and/or other stresses in order to improve its finish, consistency, appearance and/or other characteristics.
More particularly, the invention pertains to improvements in rolls for use in calenders or the like wherein a hollow cylindrical shell or sleeve surrounds a stationary carrier (normally an elongated shaft) and at least a portion of the shell consists of magnetizable material. The carrier supports one or more electromagnets which can be energized so as to cause the shell to float around the carrier. The pole faces of the electromagnets are directed toward an air gap between the internal surface of the shell and the carrier, and the roll further comprises means for applying to the electromagnets an exciting current in order to maintain the shell in a state of contact-free suspension with respect to the stationary carrier.
In a conventional roll of the above outlined character (which is used to apply pressure to flexible materials by cooperating with a counterroll), a electromagnet with two pole faces is installed in the interior of the shell opposite the nip of the two rolls. It is also known to replace such single electromagnet with a plurality of electromagnets which are connected in series. The exciting current is regulatable in order to change the pressure between the peripheral surface of the shell and the peripheral surface of the counterroll. The pole faces of the electromagnet or electromagnets extend along an arc of not more than 180 degrees and normally not in excess of approximately 90 degrees (as considered in the circumferential direction of the stationary carrier for the floating shell of the roll). The compensating means for eventual flexing of the shell includes adjustable bracing means which engages the carrier for the electromagnet(s). The end portions of the shell are mounted in bearings which are independent of or distinct from the carrier.
It is further known to construct a calender roll in such a way that the shell surrounds a stationary carrier for a series of hydrostatic supporting devices each of which establishes and maintains a cushion of pressurized fluid. The pressure of fluid in individual cushions can be regulated to thereby compensate for the tendency of certain portions of the shell to flex as a result of the action of various forces that develop in the course of a calendering operation. Regulation of pressure in the cushions can be carried out separately for each and every cushion or for groups of two or more cushions. The purpose of hydrostatic supporting devices is to insure that the pressure which the roll and the associated counterroll exert upon a web of sheet-like material is constant all the way along the full length of the nip of such rolls. The just described roll failed to gain widespread acceptance in the industry because it is very expensive and prone to malfunction. Friction between the liquid medium and the surface of the shell causes additional losses of available pressure and complicates the regulating operation.
When the hollow shell of the aforediscussed conventional roll is subjected to a very pronounced pressure which is applied by the counterroll, or when the shell is subjected to pronounced stresses under the action of the material which is treated in a machine (such as a calender) wherein the roll with a floating shell is put to use, the shell is likely to undergo a deformation which imparts thereto a substantially elliptical cross-sectional outline. Such deformation is especially likely to occur if the wall thickness of the shell is not very pronounced. On the other hand, manufacturers of calenders or like machines attempt to use shells with relatively thin walls in order to save material as well as to reduce the overall weight (this is especially important when the rolls of a calender are very long). Excessive deformation of the roll is not permissible in any of a number of various machines, such as calenders, smoothing devices, embossing devices, transporting and guiding devices for paper, sheet-like textile and/or synthetic plastic materials. Analogous problems arise as a result of excessive deformation of rolls in printing machines, rolling mills in steel plants and similar establishments wherein the material is grasped, transported, compressed and/or otherwise treated by two or more rolls during travel through the nips of neighboring rolls. For example, when the roll is used in a calender and its shell undergoes a certain amount of deformation, a web of paper or the like which passes through the nip of the roll with a counterroll is subjected to a greatly reduced pressure because, instead of being in mere linear contact with the peripheral surface of the shell and with the peripheral surface of the counterroll, the material is in substantial surface-to-surface contact with the shell as well as with the counterroll. This, in turn, entails a great reduction of pressure upon successive increments of the material. Such reduction of pressure can greatly affect the quality, appearance and/or other characteristics of a running web of paper or the like. Another drawback of the just discussed deformation of the shell is that it is not uniform along the full length of the nip of the shell with the counterroll. Therefore, the material of a web which is caused to run through the nip of the two rolls is streaky, i.e., a longitudinally extending stretch having first characteristics is adjacent to a longitudinally extending stretch whose characteristics are quite different.
A similar situation arises when a shell of the aforediscussed character is used in a guide roll wherein the web of running material is under tension and surrounds a substantial part of the peripheral surface of the shell. Furthermore, the shells of relatively long rolls which serve to transmit torque are often flexed as a result of the development of a reaction force which acts at right angles to the direction of pressure between the roll and the counterroll. This, too, influences the configuration of the gap between the two rolls and adversely affects the quality of treatment to which the running web is subjected. Eventual imbalance of the shell (such imbalance is likely to arise if the shell is deformed) or pronounced unevenness of the material of the running web (i.e., changes or fluctuations in such physical characteristics as thickness, density, composition, surface finish and/or others) can generate vibrations which initiate periodical changes in the configuration of the shell, either at certain points or along the full length of the roll.